Fixed #6168 -- Updated the Django system test runner to make it possible to run the invalid_models test (and any other invalid_ test) explicitly, rather than just as part of the full suite. Thanks for tracking down the problem, Eric Florenzano.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7576 bcc190cf-cafb-0310-a4f2-bffc1f526a37